Stimuli-sensitive hydrogels have found applications in actuators, sensors, drug delivery and bio separations. These materials are able to respond reversibly to an external stimulus that causes a distinct measurable effect on the physical properties of the material. Hydrogels are known to be sensitive to pH, ion concentration, temperature, solvent composition and electric potential. The hydrogels can be also designed to swell upon presence of a target molecule. They can be constructed in a way that the magnitude of swelling can be proportional to the concentration of ligands being present.
Minimal invasive procedures are characterized in that either through natural openings of the body or through tiny incisions inspection of the interior of the human or animal body is executed with devices such as an endoscope and a catheter. Important parameters to be measured in the human or animal body are temperature and pH level, because these are strong indicators for diseased tissue. Moreover biomarkers such as enzymes, proteins, cytokines, sugar derivates (such as e.g. glucose) and other small molecules can further help to characterize the state of the tissue.
In “An ultrasensitive chemical microsensor based on self-aligned dry-patterned environmentally sensitive hydrogels” by M Lei et al., Proc. 13th Int. Conf. Solid state sensors, actuators and microsystems, Korea, 2005, pp 1824-1827 an ultrasensitive chemical microsensor based on an environmentally sensitive hydrogel actuated cantilever beam is disclosed. The hydrogel swells in response to an increase of pH or glucose concentration and lifts and deflects the cantilever. The deflection can be detected with a variety of sensing techniques, such as optical, piezo-resistive or piezo-electric techniques. A disadvantage of the known sensor is that it is not suitable for application inside a bodily lumen, because it is not biocompatible, it is provided on a silicon substrate and it requires a separate system to detect the deflection of the cantilever.